



O V 1 



^7 







:', 





•^o« 
















4 * v v • 




v* <xy ^ 









r.s- ,G 




.> 



. . ** .0 



■a? *<• 



**.^ :iX?^ ^<y °v^»"- '-^.^ 



a V. '- 1 <y 




^ 







\*. * 






& 



.^ . o » o - <i>. 



^ ** 




^q 














-S 



<? 












A V -V : 











v^ N 




AV 














^ 



A.' 



.^^, 














^ a^ * cSkV"* ^ A" 



^ * G ... % 



A 9 S -*-"' ^ 



^ „ « u , ^ 






V v V*. . . o - -0 

V t ^oL> So a^ 

3^o 



£* ^ 



a^ *%*> • la/mr^ * «; 












/ ^,\ .^sXmxS y.vs&X .<?s£zkS.. Ss&kX 













^o< 



%^ .-ate-, xs .-issfei-. ^.^ /jster-. \„./ .-isM,-. *^** 






^ 
























^ c 



\ 



V 



W<V 












, ; ^\ °-%w ; ^ v "\ J -.^K ; ^\ °-xlw ; ^ v \ 



,0 



0' 






..... / %/^^v o °-^ 




*♦.. 



'. . » * a 







A 
















•*" 




"O V 



V 








■^ 




^°^ 





>*.,# Sj^iA-. ++.*+ /fife\ v.«^ -vcvV/b:- ^ .^ /fife-. *^ .,** .wr. ^ a^ / 






V >>'A 




Bureau of Mines Information Circular/1988 



Surface Subsidence Over Longwall Panels 
in the Western United States— Final 
Results at the Deer Creek Mine, Utah 



By Frederick K. Allgaier 




UNITED STATES DEPARTMENT OF THE INTERIOR 




Information Circular 9194 



Surface Subsidence Over Longwall Panels 
in the Western United States— Final 
Results at the Deer Creek Mine, Utah 



By Frederick K. Allgaier 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
T S Ary, Director 







Library of Congress Cataloging in Publication Data: 



Allgaier, Frederick K. 

Surface subsidence over longwall panels in the western United States. 

(Information circular; 9194) 

Includes bibliographies. 

Supt. of Docs. no. : I 28.27:9194. 

1. Mine subsidences— Utah. 2. Longwall mining— Utah. I. Title. II. Series: Infor- 
mation circular (United States. Bureau of Mines); 9194. 



TN295.U4 [TN310] 622 s [622\334] 88-600106 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Acknowledgments 2 

Deer Creek Mine study site 2 

Deer Creek Mine plan 3 

Subsidence monitoring network 6 

Subsidence monitoring surveys 6 

Subsidence profiles • 7 

Angle of draw 10 

Subsidence development 10 

Critical width 12 

Conclusions 15 

Appendix. — Measured subsidence values plotted in figures 5 and 6 17 

ILLUSTRATIONS 

1. Project location map 3 

2. Generalized overburden stratigraphy 4 

3. Deer Creek Mine plan with subsidence monuments shown as dots 5 

4. Face positions and survey dates 6 

5. Subsidence profiles 8 

6. Transverse subsidence profile 8 

7. Surface elevation profile above panel 6E 9 

8. Transverse surface elevation profile 9 

9. Subsidence contours relative to face positions 11 

10. Subsidence development for panel midpoints 12 

11. Critical mining widths from NCB and Deer Creek Mine 14 

12. Deer Creek Mine extraction width and subsidence profile 15 





UNIT OF 


MEASURE 


ABBREVIATIONS 


USED 


IN THIS 


REPORT 


ft 


feet 








pet 


percent 


in 


inch 








yr 


year 



SURFACE SUBSIDENCE OVER LONGWALL PANELS IN THE WESTERN 
UNITED STATES-FINAL RESULTS AT THE DEER CREEK MINE, UTAH 



By Frederick K. Allgaier ' 



ABSTRACT 

This report presents the final data from a 5-yr Bureau of Mines study 
designed to determine the surface subsidence characteristics resulting 
from longwall coal mining in a geologic environment common to the West- 
ern United States. It includes a description of the geologic setting of 
the study site, the mine plan, measurement techniques, and results of 
the monitoring program. Measured subsidence values were obtained over 
four adjacent longwall panels. Major subsidence characteristics such as 
longitudinal and transverse profiles, angles of draw, time-related sub- 
sidence development, and critical width are discussed. The maximum sub- 
sidence value of 5.8 ft was 68 pet of the mining thickness. The average 
angle of draw was 30°. Subsidence at the center of the first panel con- 
tinued for 46 months following undermining. The lengths of the longwall 
panels precluded a definitive determination of the critical width and 
maximum possible subsidence. 



Supervisory mining engineer, Denver Research Center, Bureau of Miaes, Denver, CO. 



INTRODUCTION 



This report is the second and con- 
cluding part of a Bureau of Mines study 
of surface subsidence characteristics 
resulting from longwall coal mining at 
the Deer Creek Mine in Emery County, UT. 
The first part was published as Bureau 
Information Circular (IC) 8896, entitled 
"Surface Subsidence Over Longwall Panels 
in the Western United States. Monitoring 
Program and Preliminary Results at the 
Deer Creek Mine, Utah." 2 The study in- 
volved the measurement of surface move- 
ments above the Deer Creek Mine, owned by 
Utah Power and Light Co. (UP&L), Salt 
Lake City, UT. Geologic and mining in- 
formation was supplied by UP&L. 

An objective of the Bureau's subsidence 
research program is to provide a means of 
predicting surface subsidence so as to 
maximize resource recovery while con- 
serving surface land values. In order 
to adequately characterize subsidence in 
the United States and develop predictive 
technology, data must first be obtained 
from a sufficient number of represen- 
tative sites. The mining and geologic 



conditions at the Deer Creek Mine are 
representative of many western coal 
mines; therefore, the results of this 
study are expected to be applicable to 
other western mining areas. 

The minimal amount of subsidence data 
available from the Western United States 
is due to a lack of research efforts. In 
many cases, data obtained by mining com- 
panies may not be available to other 
researchers. There are very few pub- 
lished case history subsidence investi- 
gations from active mining areas in the 
Western United States. This limited 
availability and distribution of subsi- 
dence information and data have hindered 
both the development of subsidence tech- 
nology, and the efforts of mining com- 
panies to comply with the Surface Mining 
Control and Reclamation Act. The data 
from this study can be used to develop 
new prediction methods or to modify ex- 
isting methods so that more accurate 
estimates can be made of subsidence under 
similar conditions in the Western United 
States. 



ACKNOWLEDGMENTS 



The Utah Power and Light Co. provided 
valuable assistance in conducting this 
research. In particular, Don Dewey, Di- 
rector of Special Projects, Chris Shin- 
gleton, Director of Technical Services, 
and Rodger Fry, Director of Exploration, 



made significant contributions to the 
project. Without the access they pro- 
vided to company property, mine plans, 
survey data, drill logs, and other infor- 
mation relating to the Deer Creek Mine, 
this study could not have been conducted. 



DEER CREEK MINE STUDY SITE 



The Deer Creek Mine is located on the 
Wasatch Plateau in central Utah approxi- 
mately 10 miles west of the town of Hunt- 
ington (fig. 1). The study site area is 
located over four adjacent longwall pan- 
els. The surface elevation of the study 
area averages 9,100 ft, and the topogra- 
phy is generally rolling, with a maximum 
ground slope of 45 pet. 



Two major coal seams are being mined in 
this area, the Blind Canyon and the 
Hiawatha. The Deer Creek Mine is located 
in the Blind Canyon coal seam which is 
approximately 50 ft above the Wilberg 
Mine in the Hiawatha coal seam. 

The overburden consists mostly of sand- 
stones and interbeds of siltstone and 
sandstone. The total percentage of 



2 Allgaier, F. K. Surface Subsidence 
Over Longwall Panels in the Western 
United States. Monitoring Program and 



Preliminary Results at the Deer 
Mine, Utah. BuMines IC 8896, 
24 pp. 



Creek 
1982, 




r 



To Price 



Carbon County 
Emery County 




Huntington 



Orangeville 



Castle Dale 



FIGURE 1. -Project location map. 



sandstone in the overburden is 45 pet, 
with 35 pet occurring in thick, beds (fig. 
2). The Deer Creek Fault is located near 



the east end of the study panels; 
however, there are no faults crossing the 
panels (fig. 3). 



DEER CREEK MINE PLAN 



The mine plan for the portion of the 
Deer Creek Mine involved in this study 
consists of four adjacent longwall panels 
that were retreat rained from east to west 
in the sequence 5E through 8E (fig. 4). 
There were room-and-pillar sections mined 
to the north (4E) and south (9E) of the 
longwall panels. To the north of panel 
5E, section 4E was mined with a total 
raining height of 10 ft. Section 4E was 
mined prior to panel 5E. Pillars were 
not rained in section 4E, and the extrac- 
tion was 52 pet. North of section 4E was 
a 200-ft-wide barrier pillar and then 
another room-and-pillar section, 3E, 
which had an extraction of 50 pet. South 



of panel 8E, a room-and-pillar section, 
9E, was mined after completion of the 
longwall panel 8E. Panel 8E was com- 
pleted in January 1983, and section 9E 
was completed in January 1984. The 
extraction in section 9E was 52 pet. 
Subsidence over panel 8E stabilized prior 
to any effect from mining in 9E. The 
four longwall panels involved in this 
study were partially undermined by long- 
wall panels in the Wilberg Mine after 
subsidence measurements were completed. 
Undermining began below panel 5E in April 
1982. The transverse line of subsidence 
monuments was undermined in November 
1982. The data presented in this report 



DEPTH, 
" 



50- 



100- 



I50-- 






250- 



300- 



350- — 



400- 



450- 



500- 



LITH0L0GY 



.•••.o..«. v* • 



• ■•■■•in 



500- 



550- 



600- 



650-- 



700- 



750- 



800 



850- 



900- 



Castlegate 
Sandstone 



LEGEND 



Interbeds 
:'.:>•":] Sandstone 
5^g Siltstone 

^f jj Mudstone 
I Coal 

vl Alluvium 



1,000- it 

1,050- ~ 



goo 



Castlegate 
Sandstone USO- f-.TTy W 



950- ^^--^rr 



1,300- 
1350- 
1.400- 



1,200-=^^= = 

f « . — • ■ . ."T 

1,250 



1450- 



i » _i j — . . 



1,500-1 

Blind Canyon 

coal seam 

1.55 OH 



Hiawatha 
coal seam 



l.^-v /:•":.' : .- v -"3 Tsi 



Star Point 
Sandstone 



FIGURE 2.-Generalized overburden stratigraphy. 



C50 
o o 



aDQQan laDaaaananoQQDDcioaaoaaaQai . 

DDQQDDl JQdDDDO section 4eQ d DaaDODa ODDQoaaaDaQai_ 

DDDQa 000000000 DODDDDODaDannDDDODaODDQOODQODaDl 

DaaOOO00OOa ODQ0DQO OODDD0£)£JgiaODOODaO00000QOOOOi 

iX „„„ , , =ai pDDr 

Panel 5E o QQj 

C30 ° pl3 C20 00 CIO 

O O OOOOOOOOOOOOOOOOOOO OOOO 9lP||0 o o 



.JdQoaa 
[QDaaaa 



^8888 

PPQaa 

dBDDd 



E50 
o o 



E40 
OOOO 



o o o o o o 



E30 
o o o o o 



o o o o o 



E20 
o o o 



o o 



r-' 1. 1! 'i innDPaanaiPaaDQ adnaaaaoaaaaaaaQ 
DQQadK^-l .=»pio 

aaaOOI Panel 6E 

DD00D 
o]£]<3BE 
DDOOof^ 

qaonouDB_ 

ddoqod 
,naDoaoi F4 o 

Jo^QO^q]! © c 



iPOQo csaczj g=^Q"ca caocaoao CJO~c3 pop 



OOOO 



Panel 7E 

F30 
o o o o o o 



DaaaDODODI 

aODoonaaoa ^l_ . - — — - - - — 

QDnaaaoaar Panel 8E 

G50 nnaDODDI G40 I 630 



pi 

8 o o o o 
°PI7 



F20 
oooo 



5CDc=>C=»C3^»J 



□5oc 



ooolooooooooooo 



620 

cPoooooooooooO 



IDDO0D 
UDaan 

QDaDD 



Pc3C3t3qoc3gta »=" 



Section 9E < 

Room-ana— 




* a Qoa e=a ea ogg 




o o 



o o o o o 



o o 



o o 



GI0 
ooooooooo 



lar extraction 




Room-and-plllar extracti 



o 



nsm- 



Scale, ft 



m 



FIGURE 3.-Deer Creek Mine plan with subsidence monuments shown as dots. 



were obtained prior to surface movements 
caused by mining in the lower seam and do 
not include any multiseam effects. 

Panel 5E was developed using three 
entries; the remaining three panels were 
developed with two entries. Entries 18 
ft wide were driven on 50-ft centers with 
crosscuts on 100-ft centers. Dimensions 
of the mined areas of the four panels are 
as follows: 5E, 480 by 2,500 ft; 6E, 540 
by 2,500 ft; 7E, 500 by 2,450 ft; and 8E, 



520 by 2,300 ft. The average depth of 
cover over the four panels was 1,500 ft. 
Mining in panel 5E began in December 
1979, and panel 8E was completed in Jan- 
uary 1983. Face positions for each month 
during the mining period are shown in 
figure 4. The average distance the face 
moved per month in the four panels was 
220 ft, including the time it took to 
move equipment between panels. 



Angle of draw 



25' 



Dec 
79 







A 




A 




A 


1 






Nov 


Oct 


Sep 




Aug 


Jul 


1 Jun 


May 




79 


79 


79 






79 


79 


I 79 


79 



Panel 
5E 



Angle of draw 
28° 



28' 



Jan 
81 



I 



Dec 
80 



nn 



Nov 
80 



Oct 
80 



Sep 
80 



u 



Aug 
80 801 



Jul Jun 



80 



May 
80 



Apr 
80 



Mar 
80 



32° 



35' 







400 



Scale, ft 



35° 
Angle of draw 



FIGURE 4.-Face positions and survey dates. 



SUBSIDENCE MONITORING NETWORK 



Feb 
80 



1 


J) 111 


1 1 


1 


II 




Jam Dec 
83 82 


Nov Oct Sep 
82 82 82 

1 1 


Aug 
82 


Jul 
82 


Jun 
I 82 


May 
82 













aa 


u 


A 


A 


A 




Mar 


Feb 


Jan 


Dec 


Nov 


Oct 


Sep 


Aug 


Jul 


Jun 


Mar 


82 


82 


82 


81 


81 


81 


81 


81 


81 


81 


81 



6E 



7E 



Feb 
81 



8E 



26° 



33 c 



28 



LEGEND 
Survey dates 



A 



The subsidence monitoring network con- 
sists of lines of survey monuments lo- 
cated over the longitudinal axis of each 
panel and a transverse monument line 
across the centers of the panels (fig. 
3). Monument spacing was 100 ft except 
over panel 8E, where 50-ft spacing was 
used over the edges of the panel. The 
purpose of the reduced spacing was to 
provide a comparison of angle-of-draw 
calculations for 50- and 100-ft monument 
spacings. 

Subsidence monuments were constructed 
of either 1-in-diam steel rods or 1.5-in- 
diam steel pipes. Monuments were driven 



to a depth of between 3 and 4 ft, 
depending on the depth of the soil cov- 
er. Approximately 6 in of the monuments 
extended above the ground to accommodate 
the survey target used in the horizontal 
position surveys. Vertical movement of 
the monuments due to frost heave or other 
local soil conditions did not exceed 
0.03 ft, the accuracy of the elevation 
surveys. This value is the average stan- 
dard deviation of the elevations for all 
nonsubsiding monuments over the duration 
of the project. Any vertical movement of 
the monuments not caused by subsidence 
was less than this value. 



SUBSIDENCE MONITORING SURVEYS 



Initial base line locations were deter- 
mined following installation of the sub- 
sidence monuments and prior to any subsi- 
dence activity. Horizontal coordinates 



for each monument were established by 
traverse survey procedures from control 
stations with known coordinates, which 
were located on stable ground beyond 



the subsidence areas. Traverse surveys 
were used only when horizontal coordi- 
nates were required in addition to eleva- 
tions. The majority of the periodic sur- 
veys conducted during subsidence activity 
needed only to establish elevations for 
the subsidence monuments. For these sur- 
veys, elevations of the monuments were 
determined using third-order direct level 
surveying techniques. 3 Closed loops were 
run from a stable control point into the 
subsidence area and then back to the 
control point. 

Periodic monitoring surveys were per- 
formed on a monthly basis while the site 
was accessible during the summer months; 
the site was typically inaccessible from 
November to June because of snow cover. 
The lack of data for these time intervals 
did not adversely affect the overall 



results of the project because adequate 
data were obtained during the summer 
months over the 5-yr monitoring period 
to establish the subsidence profiles. A 
total of 26 surveys were conducted during 
the study, including a baseline survey 
prior to mining (fig. 4). 

Data from each survey were stored as 
separate computer files, which were com- 
pared to other surveys to produce numer- 
ical records of the subsidence, such as 
coordinate or elevation differences, or 
were plotted as subsidence profiles, sub- 
sidence contours, plan plots of the moni- 
toring network, and horizontal vector 
plots. Additional details on the sur- 
veying techniques, equipment, and accura- 
cy are contained in the preliminary 
report on this study, published as Bureau 
IC 8896. 



SUBSIDENCE PROFILES 



The final longitudinal subsidence pro- 
files for panels 5E , 6E , 7E , and 8E are 
shown in figure 5, and the final trans- 
verse subsidence profile across the mid- 
points of the panel lengths is shown in 
figure 6. Data for these profiles are 
contained in the appendix. 

The maximum subsidence measured over 
the four panels was 5.8 ft, which oc- 
curred over panel 6E at station P6 on the 
transverse monument line. This point is 
approximately 200 ft north of the center 
of the four panels. The 5.8 ft repre- 
sents 68 pet of the average extraction 
height of 8.5 ft over the four panels. 

It is apparent from the transverse sub- 
sidence profile crossing the four panels 
(fig. 6) that the chain pillars between 
panels crushed out to some degree and did 
not significantly reduce the subsidence 
above the pillars. There are no charac- 
teristic undulations or bumps in the sub- 
sidence profile that would occur above 
the pillars if they remained stable. 

■^U.S. Department of Commerce. Classi- 
fication, Standards of Accuracy, and 
General Specifications of Geodetic Con- 
trol Surveys. Rock vi lie, MD, June 1980, 
12 pp. 



The maximum change in overburden depth 
due to topography is 175 ft along the 
panel lengths and 160 ft across the panel 
widths. Figure 7 shows the surface ele- 
vations over panel 6E, which is typical 
of the overburden variations over the 
four panels; maximum overburden occurs 
near the center of the panel lengths and 
decreases toward both ends. Figure 8 
shows the overburden variation across the 
panels. The maximum overburden occurs 
over panel 5E and the minimum over panel 
8E; the elevation difference is approxi- 
mately 160 ft. This difference is ap- 
proximately 11 pet of the average over- 
burden depth and should have little 
effect on the magnitude of subsidence or 
the shape of the subsidence profile. For 
example, using the National Coal Board 
(NCB) prediction method, 4 this change in 
overburden across a 1 , 500-f t-wide opening 
would change the subsidence factor by 
only 0.005. 



^National Coal Board, Production De- 
partment. Subsidence Engineers' Hand- 
book. London, 1975, 111 pp. 



C50 C45 C40 C35 C30 C25 C20 CI5 CIO C5 CI 



E50 E45 E40 E35 E30 E25 E20 EI5 EIO E5 El 




1,000 



F45 F40 F35 F30 F25 F20 FI5 FIO F5 Fl 




3,000 4,000 



5,000 




1,000 



2.000 



G50 G45 G40 G35 G30 G25 G20 GI5 GIO G5 Gl 




DISTANCE, ft 

FIGURE 5.-Subsidence profiles. A, panel 5E; B, panel 6E; C, panel 7E; D, panel 8E. Shaded areas indicate unmined coal. 




,000 



2,000 3,000 

DISTANCE,ft 



4,000 



FIGURE 6.-Transverse subsidence profile. Shaded areas indicate coal pillars. 



2 



9,180 



9,140 



9,100 



9,060 



9,020 



8,980- 



400 

I i_ 

Scale, ft 




8,94 1 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 1 I I I I I I I I I I I I I I 

E50 E45 E40 E35 E30 E25 E20 El 5 EIO E5 El 

SUBSIDENCE MONUMENTS 

FIGURE 7.-Surface elevation profile above panel 6E. 



9,240 



9,200 



9,160 



9,120 



O 9,080 



9,040- 

9,100 

8,960 



8,920 



400 



Scale, ft 



W-Panel 5E-»| |*-Panel 6E— *| k- Panel 7E— J k Panel 8E— 4 
1_iJ i i i I i i i i I i i i i I i i i i I i i i i 



J i i !'''■ I i i i i i i 



PI6 



PI2 



P7 



P2 P20 

SUBSIDENCE MONUMENTS 



P25 



P30 



P35 



FIGURE 8.-Transverse surface elevation profile. 



10 



ANGLE OF DRAW 



The angle of draw from a vertical ref- 
erence was calculated for each end of the 
four panels and for the transverse monu- 
ment line extending beyond the south edge 
of panel 8E. The calculated angles of 
draw are shown on figure 4. 

Angle-of-draw calculations are very 
sensitive to the accuracy of the surveys 
used to measure vertical movement. 
Assuming the overburden depth and the 
location of the limit of mining are 
known, the angle of draw is dependent on 
locating the point of zero subsidence. 
The more accurate the surveys, the far- 
ther the point of apparent zero subsi- 
dence moves away from the mined area, 
thus resulting in larger calculated 
angles of draw. Conversely, less accu- 
rate surveys result in smaller angles of 
draw. Elevations used to calculate 
angles of draw were determined by trigo- 
nometric and differential leveling. The 
average standard deviation for elevations 
of nonsubsiding monuments was 0.08 ft for 
trigonometric leveling and 0.03 ft for 
differential leveling. 

Another factor that can affect the 
angle-of-draw calculations is the spacing 
of the subsidence monuments. Monument 
spacing was 100 ft over panels 5E, 6E, 



and 7E and 50 ft over panel 8E. The 
effect of reduced monument spacing is to 
reduce the distance over which the point 
of zero subsidence is interpolated. Re- 
duced monument spacing can improve the 
accuracy of the angle-of-draw determina- 
tion, but will not tend to increase or 
decrease its magnitude. The average of 
the eight angles of draw beyond the ends 
of the panels, and the one angle of draw 
at the end of the P line of monuments 
south of panel 8E is 30*. 

At the east ends of the panels, the 
subsidence profiles cross the Deer Creek 
Fault. The expected effect of the fault 
would be to reduce the angles of draw. 5 
A comparison of the angles on either 
ends of the panels does not support this 
expectation. On two panels, the angles 
are smaller on the east ends, and on the 
other two panels, the angles are smaller 
on the west ends. Although there is no 
major effect on the angle-of-draw values, 
the fault did change the shape of the 
subsidence profiles. On each of the four 
longitudinal profiles, there is a dis- 
tinct step or increase in slope of the 
profiles between monuments 10 and 13 
(fig. 5). 



SUBSIDENCE DEVELOPMENT 



Over the first mined panel, 5E, sub- 
sidence did not occur at the surface un- 
til the face had retreated between 550 
and 1,050 ft. Subsidence continued to 
occur over this panel for approximately 4 
yr as the three panels to the south were 
mined. The progression of subsidence 
over the four panels as a function of 
face position is shown on a series of 
subsidence contour plots (fig. 9). Fig- 
ure 9F represents the final subsidence 
values. 

The timing, rate, and duration of sub- 
sidence over the four panels is illus- 
trated in figure 10. The subsidence 
values near the midpoint of each panel 
length (monument 27) are plotted against 
time. Initial subsidence at the mid- 
points of the first and second panels 



mined occurred at 3 and 2 months, respec- 
tively, after the points were undermined. 
Initial subsidence at the midpoints of 
the third and fourth panels mined pre- 
ceded undermining by 3 and 11 months, 
respectively. Subsidence at the midpoint 
of panel 8E began as the face passed 
under the midpoint of the adjacent panel, 
7E. 

The length of time from undermining to 
final subsidence became progressively 
shorter from the first through the last 
panel. It took 46 months for monument 
C27 to reach final subsidence after 
it had been undermined, while monument 

5 Lee, A. J. The Effect of Faulting on 
Mining Subsidence. Min. Eng., London, 
v. 125, 1965-66, pp. 735-745. 



11 



Panel 
5E 



6E 



7E 



8E 




A September 1979 

0.4 1.2 2.0 



Panel 
5E 



1.6 0.8 



6E 



7E 



8E 




C November 1980 



1.0 2.0 



2.0 1.0 




SJuly 1980 



0.5 1.0 1.5 2.0 



8E 



D March 1981 



1 .0 2.0 3.0 



2.0 1.5 1.0 0.5 




3.0 2.0 1.0 




E June 1982 F January 1983 

FIGURE 9.-Subsidence contours relative to face positions. Shaded areas indicate mined portions of the panels. 



12 




12 months 



20.5 months 



36 months 



i ' ' i i ' ' 



J_L 



46 months 



i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 



i i i i i 



' ' ' ' ' ' ' ' 



1979 



1980 1981 1982 

TIME, months 

FIGURE 10.-Subsidence development for panel midpoints. 



1 983 



G27 reached final subsidence in only 
12 months. Although the elapsed time 
between undermining and final subsidence 
varied greatly, the midpoints stabilized 



at the final subsidence values at nearly 
the same point in time, between July and 
September 1983. 



CRITICAL WIDTH 



An important parameter to be investi- 
gated in this study was the critical 
extraction width. The critical width is 
the minimum dimension of an extraction 
area required to cause maximum possible 
subsidence in the center of the area, and 
is represented as a function of the seam 
depth. For instance, in British coal- 
fields the critical width is 1.4 times 



the depth of the seam. 6 When an ex- 
traction area increases and the minimum 
dimension reaches the critical width, the 
point at the center of subsidence profile 
will reach maximum possible subsidence. 
As the minimum dimension of the extrac- 
tion area exceeds the critical width and 

6 Work cited in footnote 4. 



13 



becomes supercritical, the subsidence 
profile assumes a characteristic flat- 
bottomed shape, with more than one point 
reaching maximum possible subsidence. 

The extraction area at the Deer Creek 
study site is nearly square. The extrac- 
tion width across the four panels, 2,400 
ft, is approximately equal to the aver- 
age panel length of 2,432 ft. The shape 
of the transverse subsidence profile 
(fig. 6) indicates that the Deer Creek 
Mine extraction area is subcritical. 
There is no indication that the profile 
is beginning to flatten out near the 
center; therefore, the critical width is 
greater than the final extraction width 
of 2,400 ft. Using the average over- 
burden depth of 1,500 ft, the critical 
width is at least 1.6 times the depth. 

Further mining to the south would in- 
crease the extraction width beyond the 
panel lengths, and the transverse profile 
would have a flat bottom; however, the 
subsidence at the center of the panels 
would continue to be constrained by the 
length of the panels and would not be 
expected to increase beyond the maximum 
measured value of 5.8 ft. Because the 
panel lengths are not greater than the 
extraction width required to cause the 
transverse profile to flatten out, a 
determination of the critical width, and 
thus, the maximum possible subsidence 
cannot be made. Mining did occur south 
of the study panels in the 9E room-and- 
pillar section. During the time 9E was 
being mined, the center of the transverse 
profile was affected by longwall mining 
in the Wilberg Mine below the Deer Creek 
Mine; thus, it was not possible to dif- 
ferentiate the subsidence due to each 
mining operation. 

The relationship between the angle of 
draw and critical width parameters from 
this site results in a contradiction when 
compared to existing definitions. Ac- 
cording to the NCB, 7 the angle of draw 

7 Work cited in footnote 4. 



can be used to define the critical width; 
the critical width should be directly 
proportional to the angle of draw (fig. 
1L4). At the Deer Creek Mine site, this 
relationship is not evident (fig. 11B). 
In the NCB case, the angle of draw over 
solid, unmined coal can be projected over 
the caved panel to define the critical 
width. At the Deer Creek Mine site, the 
angle of draw over solid coal (30°) is at 
least 8° less than the angle over the 
caved panel required to define the criti- 
cal width (38.6°). 

One explanation for this condition is 
that the Deer Creek Mine extraction area 
is critical or supercritical. This pos- 
sibility is supported by the fact that 
the angle required to reach the point of 
maximum subsidence, P6, on the transverse 
profile from the north edge of the ex- 
traction area is within 1.5° of the aver- 
age calculated angle of draw (fig. 12). 
If this is the case, the points on the 
profile for a distance of 475 ft south of 
point P6 toward PI should also reach the 
maximum subsidence value of 5.8 ft and 
produce the flat-bottomed profile charac- 
teristic of a supercritical extraction 
area. The maximum additional subsidence 
required for this condition, As, is 0.5 
ft (fig. 12). This would mean that a 
condition existed that prevented an area 
over panels 6E and 7E from reaching maxi- 
mum subsidence. 

Strata bridging in one of the massive 
sandstone layers in the overburden, such 
as the Castlegate Sandstone, is one pos- 
sible explanation for a delay in subsi- 
dence at this location. This cannot be 
confirmed, because there was no instru- 
mentation in the overburden and because 
the subsidence from subsequent, adjacent 
mining was combined with that from mining 
in a lower seam. A finite-element analy- 
sis of the Deer Creek Mine conditions and 
overburden stresses indicated that a 
strata bridge could support the over- 
burden load across one panel width. 



14 



Ground surface 




A, National Coal Board 



Ground surface 




B, Deer Creek Mine 

FIGURE 11. -Critical mining widths from NCB (A) and Deer Creek Mine (£). 



Another possible cause of incomplete 
subsidence could be a minimal amount of 
support from the chain pillars. The 
location of these chain pillars relative 
to the transverse subsidence profile 
(fig. 6) is such that they could be pre- 
venting the small amount of additional 
subsidence necessary to produce the 



characteristic flat-bottomed profile of a 
supercritical extraction area. Again, 
because no additional panels were mined 
to the south before the subsidence was 
influenced by mining in the lower seam in 
the Wilberg Mine, this possible cause 
cannot be confirmed. 



15 



P25 




FIGURE 12.-Deer Creek Mine extraction width and subsidence profile. 



CONCLUSIONS 



The final maximum subsidence factor was 
0.68, although it cannot be confirmed 
that this value represents the maximum 
possible subsidence because the extrac- 
tion dimensions precluded determination 
of the critical width. 

The time required to reach final sub- 
sidence after undermining can be as much 
as 4 yr owing to adjacent panel effects. 

The average angle of draw was 30°. 
This value can be used to define the 
limit of subsidence beyond the limit of 
mining, but it cannot be used to deter- 
mine the critical width. 



A major fault increased the slope of 
the subsidence profile in the vicinity of 
the fault, but did not reduce the lateral 
extent of subsidence. However, the mag- 
nitude of subsidence beyond the fault was 
reduced. 

An area of 315 acres was gradually low- 
ered by mining of the four longwall 
panels. During the 4 yr that subsidence 
was monitored at this site, there was no 
evidence of damage to the land surface, 
vegetation, or drainage patterns. There 
was no visual indication of any surface 
depressions, and no surface cracks that 



16 



would adversely affect the foreseeable 
value or use of this land* The absence 
of tension cracks can be attributed, in 
part, to the fact that the chain pillars 
between panels did not remain stable, 
which would have caused an uneven subsi- 
dence profile across the panels. 

Any continuation of this research 
should begin by making a detailed compar- 
ison of these results to other case stud- 
ies in an attempt to isolate the mining 
or geologic variables controlling specif- 
ic aspects of the resulting subsidence. 
Future studies should be designed to re- 
fine the angle-of-draw estimates obtained 



in this study, define the critical ex- 
traction width, which is needed to deter- 
mine the area necessary to cause maximum 
possible subsidence, and obtain meaning- 
ful and accurate strain values. These 
are the parameters that are the most 
important for predicting the magnitude 
and extent of possible adverse effects of 
subsidence from future mining. Once 
these parameters are known, efforts can 
be made to eliminate or mitigate these 
effects through the engineering design of 
mines and of surface facilities that may 
be subject to subsidence. 



APPENDIX. --MEASURED SUBSIDENCE VALUES PLOTTED IN FIGURES 5 AND 6 



17 



Subsi- 



Subsi- 



Subsi- 



Monument 


dence , 
ft 


Monument 


dence , 
ft 


Monument 


dence, 
ft 


Monument 


dence, 


Monument 


dence, 








ft 


ft 




0.1 




0.0 




0.1 




0.0 




2.2 




.1 




.0 




• 1 




.1 




2.6 




.2 




.0 




• X 




.1 




3.1 




.2 




.0 




• X 




.1 




3.6 




.2 




.0 




• X 




.1 




4.7 




.2 




.0 




• X 




.1 




4.9 




.3 




.1 




.2 




.1 




5.1 




.3 




.1 




.2 




.1 




5.3 




.4 




.1 




.2 




.1 




5.4 




.4 




.1 




.2 




.2 




5.7 




.6 




.2 




.2 




.2 




5.8 




.9 




.5 




.3 




.4 




5.7 




2.3 




.9 




.6 




.7 




5.6 




2.6 




2.3 




1.8 




1.6 




5.7 




2.9 




2.8 




2.5 




2.0 




5.4 




3.2 




3.1 




2.9 




2.1 




5.1 




3.6 




3.4 




3.5 




2.3 




4.7 




3.8 




3.6 




4.3 




2.6 




4.6 




4.0 




3.9 




4.9 




3.0 




4.4 




4.0 




4.1 




5.2 




3.2 




4.2 




4.1 




4.2 




5.2 




3.4 




3.9 




4.2 




4.4 




5.2 




3.4 




3.4 




4.2 




4.6 




5.2 




3.5 




2.8 




4.3 




4.8 




5.1 




3.5 




2.2 




4.2 




4.9 




5.1 




3.4 




1.9 




4.2 




5.0 




5.0 




3.4 




1.6 




4.2 




5.1 




5.0 




3.3 




1.3 




4.2 




5.2 




4.9 




3.2 




.8 




4.1 




4.9 




4.8 




3.0 




.5 




4.2 




4.9 




4.8 




2.7 


P30 


.3 




4.0 




4.8 




4.7 




2.5 




.2 




3.9 




4.7 




4.5 




2.2 




.1 




3.6 


E33 


4.4 




4.2 




2.0 




.1 




2.9 




3.8 




3.6 




1.6 




.1 




2.1 




3.0 




2.6 


G35 


1.2 




.0 




1.6 




2.0 




1.8 




.8 




.0 




1.1 




1.2 




1.2 




.6 




.0 




.8 




.8 




.8 




.4 








.6 


E39 


.6 




.7 




.2 








.4 




.5 




.5 




.2 








.3 




.4 




.4 




.2 








.2 




.4 




.4 




.2 








.1 




.3 




.3 




.1 








.1 




.2 




.3 




.0 








.0 




.2 




.2 




.1 








.0 




.1 




.2 




.1 








.1 




.1 




.1 




.1 








.1 




.1 








.0 








.0 




.0 








.0 








.0 




.0 








.0 







Subsi- 



Subsi- 



U.S. GOVERNMENT PRINTING OFFICE: 1988 — 547-000/80,066 



INT.-BU.OF MINES,PGH.,PA. 28754 



U.S. Department of the Interior 
Bureau of Mines— Prod, end Distr. 
Cochran* Mill Roed 
P.O. Box 18070 
Pittsburgh, Pa. 15236 



AN EQUAL OPPORTUNITY EMPLOYER 



OFFICIAL BUSINESS 
PENALTY FOR PRIVATE USE. $300 

"2 Do not wi sh to recei ve thi s 
material, please remove 
from your mailing list* 

] Address change* Please 
correct as indicated* 



\* 



•6°i 



o *.„.' 0' 








a .**S§&**. ^ a* /, 




> 






\^ .-ate-, V* 



















o O A* 
















^^ 











/K- "*^ T .*, 











r^ 







a.* ~- ^ 






J ^v 



o . > * A 











*V 



4 -V ^ 



» ^ 












<£ ^ 







'o^ 



^ 




0' 




,♦♦ .-ate-. X^ .vSSte %,** .-Jfe-- \/ .^ 







^CKMAN 
INDERY INC. |§ 

^ NOV 88 

g^ N. MANCHESTER, 
- INDIANA 46962 



•- >..»* ■•iRSA 



^ V" 

maBeaaainiiiiiiiiiiii^ 



